Electrostatic energy harvester

ABSTRACT

An electrostatic energy harvester broadly comprises an electrical energy storage component, an electrical energy transfer stage, first and second variable capacitors, and a switching control module. The electrical energy transfer stage includes diode-connected transistors and dictates electrical energy transfer between the electrical energy storage component and the variable capacitors. The switching control module timely switches between the first and second variable capacitors according to a state machine. Subsequent electrical energy investments from the electrical energy storage component are less than an initial electrical energy investment due to remnant electrical energy remaining at the previously active one of the first and second variable capacitors from previous electrical energy harvesting.

GOVERNMENT INTERESTS

This invention was made with Government support under Contract No.:DE-NA-0002839 awarded by the United States Department of Energy/NationalNuclear Security Administration. The Government has certain rights inthe invention.

BACKGROUND

Electrostatic energy harvesters are used to convert mechanicalvibrational energy into electrical energy for powering low-energyelectronic components such as remote or isolated sensors, implantedmedical devices, and wearable electronics. Conventional electrostaticenergy harvesters produce nanojoules of energy per vibration cycle,while most electrostatic energy harvesting applications requiremillijoules of energy per vibration cycle. As such, severalelectrostatic energy harvesters are needed to power even a singlelow-energy electronic component.

Electrostatic energy harvesters have either synchronous electricalenergy transfer stages or asynchronous electrical energy transferstages, both of which have drawbacks. Synchronous electrical energytransfer stages have high current leakage and require complex controlcircuitry. Asynchronous electrical energy transfer stages require twobatteries and have a limited energy harvesting window in which toharvest electrical energy over the course of a vibration cycle.

Furthermore, dual variable capacitors have been proposed to double theenergy harvested per vibration cycle. However using dual variablecapacitors as previously proposed in literature would requireduplicating and adding to already complicated switching control modulessuch as timing circuits, which result in additional significant energylosses.

SUMMARY

Embodiments of the invention solve the above-mentioned problems andother problems and provide a distinct advancement in the art ofelectrostatic energy harvesters. More particularly, the inventionprovides more efficient electrical energy harvesting and less complexelectrical energy transfer stages and variable capacitor switchingcontrol.

An embodiment of the invention is an electrostatic energy harvesterbroadly comprising an electrical energy storage component, an electricalenergy transfer stage, first and second variable capacitors, and controlmodules. The electrostatic energy harvester retains remnant electricalenergy from each energy harvesting phase on an active one of the firstand second variable capacitors to reduce subsequent electrical energyinvestments.

The electrical energy storage component is connected to the electricalenergy transfer stage and is a battery, a large charged capacitor, asuper capacitor, or the like. The electrical energy storage componentprovides an initial electrical energy investment and subsequentelectrical energy investments to the first and second variablecapacitors.

The electrical energy transfer stage is connected between the electricalenergy storage component and the first and second variable capacitors.The electrical energy transfer stage includes switches or otherconventional electrical or electronic components and dictates electricalenergy transfer between the electrical energy storage component and thefirst and second variable capacitors.

The first variable capacitor is connected to the electrical energytransfer stage via a high node and to a ground node or ground plane viaa first grounding switch. The high node may include an oscillating mass,which may be shared with the second variable capacitor.

The second variable capacitor is connected to the electrical energytransfer stage via the high node and to a ground node or ground planevia a second grounding switch. The first and second variable capacitorsmay cooperatively form an in-plane microelectromechanical (MEM) dualvariable capacitor and may be 180 degrees out of phase of each other.The grounding switches are never concurrently closed and hence the firstand second variable capacitors are never concurrently grounded or“active”.

In use, the electrostatic energy harvester converts mechanicalvibrations to electrical energy via oscillations of the first and secondvariable capacitors. In one example, the first variable capacitor isinitially active. First, an initial electrical energy investment (i.e.,a pre-charge) is provided from the electrical energy storage componentto the high node (and hence to the first variable capacitor).

External vibrations induce oscillations of the high node oscillatingmass, resulting in capacitance of the first variable capacitoralternating between increasing and decreasing. As capacitance of thefirst variable capacitor decreases, electrical energy may be harvestedfrom the first variable capacitor to the electrical energy storagecomponent. A remnant (unharvested) electrical energy may remain at thehigh node.

As the oscillation continues, capacitance of the first variablecapacitor begins to increase while voltage of the first variablecapacitor begins to decrease. The first and second variable capacitorsare switched so that the second variable capacitor is active and thefirst variable capacitor is inactive.

A subsequent electrical energy investment may then be provided from theelectrical energy storage component to the high node (and hence to thesecond variable capacitor). The subsequent electrical energy investmentmay be less than the initial electrical energy investment due to theremnant electrical energy remaining at the high node from the previouselectrical energy harvesting.

The above steps may be repeated, alternating between the second variablecapacitor and the first variable capacitor. In this way, electricalenergy is invested and harvested twice per vibration period.

The above-described electrostatic energy harvester provides severaladvantages. For example, the electrostatic energy harvester utilizesremnant energy retained on the active variable capacitor from eachenergy harvesting phase to reduce subsequent electrical energyinvestments. The remnant energy does not need to be transferred to theelectrical energy storage component, which reduces energy losses andsimplifies the electrical energy transfer stage.

Another embodiment of the invention is an electrostatic energy harvesterbroadly comprising an electrical energy storage component, an electricalenergy transfer stage, and first and second variable capacitors. Theelectrostatic energy harvester is similar to the electrostatic energyharvester described above except the electrical energy transfer stageutilizes diode-connected transistors, which eliminates severalshortcomings of conventional synchronous and asynchronous electricalenergy transfer stages.

The electrical energy storage component is connected to the electricalenergy transfer stage and is a battery, a large charged capacitor, asuper capacitor, or the like. The electrical energy storage componentprovides an initial electrical energy investment and subsequentelectrical energy investments to the first and second variablecapacitors.

The electrical energy transfer stage is connected between the electricalenergy storage component and the first and second variable capacitorsand broadly comprises a first diode-connected transistor, an inductor,and a second diode-connected transistor. The electrical energy transferstage dictates electrical energy transfer between the electrical energystorage component and the first and second variable capacitors.

The first diode-connected transistor is connected to the electricalenergy storage component and allows electrical energy to be investedfrom the electrical energy storage component to an active one of thefirst and second variable capacitors during an electrical energyinvestment phase. The first diode-connected transistor may be adepletion mode gallium nitride transistor.

The inductor is connected between the first diode-connected transistorand the first and second variable capacitors. The inductor slows downthe flow of charge from the electrical energy storage component to thefirst and second variable capacitors during the electrical energyinvestment phase. The inductor may be omitted at the cost of certainenergy losses.

The second diode-connected transistor is connected to the first andsecond variable capacitors and to the electrical energy storagecomponent and allows electrical energy to be harvested from an activeone of the first and second variable capacitors to the electrical energystorage component during an electrical energy harvesting phase. Thesecond diode-connected transistor may be a depletion mode galliumnitride transistor.

The first variable capacitor is connected to the electrical energytransfer stage via a high node and to a ground node or ground plane viaa first grounding switch. The high node may include an oscillating mass.The first variable capacitor shares the high node with the secondvariable capacitor.

The second variable capacitor is connected to the electrical energytransfer stage via the high node and to a ground node or ground planevia a second grounding switch. The first and second variable capacitorsmay be part of an in-plane microelectromechanical (MEM) dual variablecapacitor and may be 180 degrees out of phase of each other. Thegrounding switches are never concurrently closed and hence the first andsecond variable capacitors are never concurrently grounded or “active”.

In use, the electrostatic energy harvester converts mechanicalvibrations to electrical energy via oscillations of the first and secondvariable capacitors. The first variable capacitor is initially active.An initial electrical energy investment (i.e., a pre-charge) is providedfrom the electrical energy storage component to the high node (and henceto the first variable capacitor).

External vibrations induce oscillations of the high node oscillatingmass, resulting in capacitance of the first variable capacitoralternating between increasing and decreasing. As capacitance of thefirst variable capacitor decreases, electrical energy may be harvestedfrom the first variable capacitor to the electrical energy storagecomponent via the second diode-connected transistor. A remnant(unharvested) electrical energy may remain at the high node.

As the oscillation continues, capacitance of the first variablecapacitor begins to increase while voltage of the first variablecapacitor begins to decrease. The first and second variable capacitorsare switched so that the second variable capacitor is active and thefirst variable capacitor is inactive.

A subsequent electrical energy investment may then be provided from theelectrical energy storage component to the high node (and hence to thesecond variable capacitor) via the first diode-connected transistor oncethe voltage at the high node reaches a low voltage. The subsequentelectrical energy investment may be less than the initial electricalenergy investment due to the remnant electrical energy remaining at thehigh node from the previous electrical energy harvesting.

The above steps may be repeated, alternating between the second variablecapacitor and the first variable capacitor. In this way, electricalenergy is invested and harvested twice per vibration period.

The above-described electrostatic energy harvester provides severaladvantages. For example, the first and second diode-connectedtransistors eliminates the need for control circuitry between theelectrical energy storage component and the dual variable capacitor. Thefirst and second diode-connected transistors also eliminate high currentleakage, dual batteries, and energy harvesting limits based onharvesting time lost while variable capacitor voltage increases.

Another embodiment of the invention is an electrostatic energy harvesterbroadly comprising an electrical energy storage component, an electricalenergy transfer stage, first and second variable capacitors, first andsecond grounding switches, and a switching control module.

The electrical energy storage component is connected to the electricalenergy transfer stage and is a battery, a large charged capacitor, asuper capacitor, or the like. The electrical energy storage componentprovides an initial electrical energy investment and subsequentelectrical energy investments to the first and second variablecapacitors.

The electrical energy transfer stage is connected between the electricalenergy storage component and the first and second variable capacitors.The electrical energy transfer stage may include switches or otherconventional electrical or electronic components and dictates electricalenergy transfer between the electrical energy storage component and thefirst and second variable capacitors.

The first variable capacitor is connected to the electrical energytransfer stage via a high node and to a ground node or ground plane viaa first grounding switch. The high node may include an oscillating mass.The first variable capacitor shares the high node with the secondvariable capacitor.

The second variable capacitor is connected to the electrical energytransfer stage via the high node and to a ground node or ground planevia a second grounding switch. The first and second variable capacitorsmay cooperatively form an in-plane microelectromechanical (MEM) dualvariable capacitor and may be 180 degrees out of phase of each other.The grounding switches are never concurrently closed and hence the firstand second variable capacitors are never concurrently grounded or“active”.

The switching control module broadly comprises a buffer, a comparator, acontrol module resistor, a control module capacitor, a memory element,and an inverter. The switching control module timely switches betweenthe first and second variable capacitors.

The buffer is connected to the high node of the first and secondvariable capacitors. The buffer shields against high impedance input tothe comparator and may be omitted if the comparator itself hassufficiently high input impedance.

The comparator is connected to the buffer or to the high node of thefirst and variable capacitors if the buffer is omitted. The comparatoridentifies whether a voltage of the active variable capacitor (of thefirst and second variable capacitors) is rising or falling.

The control module resistor is connected between input nodes of thecomparator. The control module capacitor is connected to one of theinput nodes of the comparator. The control module resistor and thecontrol module capacitor cooperatively form a signal delay RC circuit tooutput a delayed voltage signal to the comparator.

The memory element is connected between the output node of thecomparator, and the first grounding switch and the inverter. The memoryelement may include logic table XOR gates, a latch, a flip-flop, or thelike. The memory element eliminates the need for timing circuits foridentifying which grounding switch is initially closed and ensuring thecorrect grounding switch is closed for each phase of operation.

The inverter is connected to the output side of the memory element andbetween the grounding switches. The inverter ensures the state ofgrounding switches are opposite each other and thus ensures the firstand second variable capacitors are never concurrently grounded or“active”. The inverter may be omitted if the memory element providesthis functionality. For example, a D flip flop has two outputs, Q and Qbar. Q bar is connected to one switch and Q is connected to the other.

In use, the electrostatic energy harvester converts mechanicalvibrations to electrical energy via oscillations of the first and secondvariable capacitors. The first variable capacitor is initially active.An initial electrical energy investment (i.e., a pre-charge) is providedfrom the electrical energy storage component to the high node (and henceto the first variable capacitor).

External vibrations induce oscillations of the high node oscillatingmass, resulting in capacitance of the first variable capacitoralternating between increasing and decreasing. As capacitance of thefirst variable capacitor decreases, electrical energy may be harvestedfrom the first variable capacitor to the electrical energy storagecomponent. A remnant (unharvested) electrical energy may remain at thehigh node.

As the oscillation continues, capacitance of the first variablecapacitor begins to increase while voltage of the first variablecapacitor begins to decrease. The first and second variable capacitorsare switched so that the second variable capacitor is active and thefirst variable capacitor is inactive. The switching control modulerecognizes the decrease in voltage of the first variable capacitor andhot switches to the second variable capacitor. That is, the switchingcontrol module deactivates the first variable capacitor and activatesthe second variable capacitor. Voltage at the high node dropsdramatically once the first and second variable capacitors are switched.

A subsequent electrical energy investment may then be provided from theelectrical energy storage component to the high node (and hence to thesecond variable capacitor). The subsequent electrical energy investmentmay be less than the initial electrical energy investment due to theremnant electrical energy remaining at the high node from the previouselectrical energy harvesting.

The above steps may be repeated, alternating between the second variablecapacitor and the first variable capacitor. In this way, electricalenergy is invested and harvested twice per vibration cycle.

The above-described electrostatic energy harvester provides severaladvantages. For example, hot switching the first and second variablecapacitors eliminates energy losses by keeping remnant energy at thedual variable capacitor. The switching control module, which controlshot switching of the first and second variable capacitors, reducescomplexity of variable capacitor control circuitry.

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the detaileddescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter. Other aspectsand advantages of the present invention will be apparent from thefollowing detailed description of the embodiments and the accompanyingdrawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Embodiments of the present invention are described in detail below withreference to the attached drawing figures, wherein:

FIG. 1 is a circuit diagram of an electrostatic energy harvesterconstructed in accordance with an embodiment of the invention;

FIG. 2 is an in-plane dual variable capacitor constructed in accordancewith an embodiment of the invention;

FIG. 3 is a flow diagram showing certain method steps for harvestingelectrical energy in accordance with another embodiment of theinvention;

FIG. 4 is a flow diagram of a state machine of a switching controlmodule memory element constructed in accordance with an embodiment ofthe invention;

FIG. 5 is a flow diagram of a state machine of a switching controlmodule flip-flop constructed in accordance with an embodiment of theinvention;

FIG. 6 is a circuit diagram of a flip-flop constructed in accordancewith an embodiment of the invention;

FIG. 7 is a circuit diagram of an electrostatic energy harvesterconstructed in accordance with another embodiment of the invention; and

FIG. 8 is a circuit diagram of a portion of an electrostatic energyharvester constructed in accordance with another embodiment of theinvention.

The drawing figures do not limit the present invention to the specificembodiments disclosed and described herein. The drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following detailed description of the invention references theaccompanying drawings that illustrate specific embodiments in which theinvention can be practiced. The embodiments are intended to describeaspects of the invention in sufficient detail to enable those skilled inthe art to practice the invention. Other embodiments can be utilized andchanges can be made without departing from the scope of the presentinvention. The following detailed description is, therefore, not to betaken in a limiting sense. The scope of the present invention is definedonly by the appended claims, along with the full scope of equivalents towhich such claims are entitled.

In this description, references to “one embodiment”, “an embodiment”, or“embodiments” mean that the feature or features being referred to areincluded in at least one embodiment of the technology. Separatereferences to “one embodiment”, “an embodiment”, or “embodiments” inthis description do not necessarily refer to the same embodiment and arealso not mutually exclusive unless so stated and/or except as will bereadily apparent to those skilled in the art from the description. Forexample, a feature, structure, act, etc. described in one embodiment mayalso be included in other embodiments, but is not necessarily included.Thus, the current technology can include a variety of combinationsand/or integrations of the embodiments described herein.

Turning to FIGS. 1 and 2, an electrostatic energy harvester 10 isillustrated. The electrostatic energy harvester 10 broadly comprises anelectrical energy storage component 12, an electrical energy transferstage 14, a first variable capacitor 16, a second variable capacitor 18,a first grounding switch 20, a second grounding switch 22, and aswitching control module 24.

The electrical energy storage component 12 may be connected to theelectrical energy transfer stage 14 and may be a battery, a largecharged capacitor, a super-capacitor, or any other suitable electricalenergy storage device. The electrical energy storage component 12 may beconfigured provide an initial electrical energy investment andsubsequent electrical energy investments to the first and secondvariable capacitors 16, 18 and to store electrical energy harvested fromthe first and second variable capacitors 16, 18.

The electrical energy transfer stage 14 may be connected between theelectrical energy storage component 12 and the first and second variablecapacitors 16, 18 and broadly comprises a first diode-connectedtransistor 26, an inductor 28, and a second diode-connected transistor30. The electrical energy transfer stage 14 dictates electrical energytransfer between the electrical energy storage component 12 and thefirst and second variable capacitors 16, 18.

The first diode-connected transistor 26 may be connected to theelectrical energy storage component 12 via its cathode and to theinductor 28 via its anode. The first diode-connected transistor 26 maybe a depletion mode gallium nitride transistor.

The inductor 28 may be connected between the anode of the firstdiode-connected transistor 26 and the first and second variablecapacitors 16, 18. The inductor 28 slows down the flow of charge fromthe electrical energy storage component 12, reducing the current throughthe first diode-connected transistor 26, thus reducing energy loss inthe form of heat. The inductor 28 can be omitted at the cost of(additional) energy losses in the form of heat resulting in a lessefficient electrostatic energy harvester.

The second diode-connected transistor 30 may be connected to theelectrical energy storage component 12 via its anode and to the firstand second variable capacitors 16, 18 via its cathode. The seconddiode-connected transistor 30 may be a depletion mode gallium nitridetransistor.

The first variable capacitor 16 may be connected to the inductor 28opposite the first diode-connected transistor 26 (or to the anode of thefirst diode-connected transistor 26 if the inductor 28 is omitted) via ahigh node 32 and to the first grounding switch 20 via a lownode/grounding electrode. The high node 32 may be a shared node that thefirst variable capacitor 16 shares with the second variable capacitor18. In one embodiment, the first variable capacitor 16 may be part of anin-plane microelectromechanical (MEM) dual variable capacitor 34 withthe high node 32 (i.e., the shared node) being an oscillating/vibrating(i.e., “floating”) electrode and the low node being a stationary comb36.

The second variable capacitor 18 may be connected to the inductor 28opposite the first diode-connected transistor 26 (or to the anode of thefirst diode-connected transistor 26 if the inductor 28 is omitted) via ahigh node and to the second grounding switch 22 via a low node/groundingelectrode. The high node of the second variable capacitor 18 may be theshared node (high node 32) described above. In one embodiment, thesecond variable capacitor 18 may be part of the in-plane MEM dualvariable capacitor 34 described above, with the low node of the secondvariable capacitor 18 being a stationary comb 38.

The first grounding switch 20 may be connected between the low node ofthe first variable capacitor 16 and a grounding plane or grounding node.The first grounding switch 20 may be a relay or any other suitableelectronic switch.

The second grounding switch 22 may be connected between the low node ofthe second variable capacitor 18 and a grounding plane or groundingnode. The second grounding switch 22 may be a relay or any othersuitable electronic switch. In one embodiment, the first and secondgrounding switches 20, 22 are never concurrently closed and hence thefirst and second variable capacitors 16, 18 are never concurrentlygrounded or “active”.

The switching control module 24 comprises a buffer 40, a comparator 42,a control module resistor 44, a control module capacitor 46, a memoryelement 48, and an inverter 50. The switching control module 24 timelyswitches between the first and second variable capacitors 16, 18. Morespecifically, the switching control module 24 is configured to togglethe first grounding switch 20 and the second grounding switch 22 when avoltage drop at the high node 32 is detected so as to switch between thefirst variable capacitor 16 and the second variable capacitor 18regardless of which of the first grounding switch 20 and the secondgrounding switch 22 is closed, including on initial operation.

The buffer 40 may be connected to the high node 32 and to a first inputnode of the comparator 42. The buffer 40 shields against high impedanceinput to the comparator 42. The buffer 40 also reduces an amount of lostcharge due to the inclusion of the switching control module 24. Thebuffer 40 may be omitted if the comparator 42 itself has sufficientlyhigh input impedance.

The comparator 42 includes a first input node, a second input node, andan output node. The first input node may be an inverting input node(e.g., V−) and may be connected to the buffer 40 or to the high node 32of the first and second variable capacitors 16, 18 if the buffer 40 isomitted. The second input node may be a non-inverting input (e.g., V+)and may be connected to the control module resistor 44 opposite thebuffer 40 or high node 32 and to the control module capacitor 46. Theoutput node of the comparator may be connected to the memory element 48.

The control module resistor 44 may be connected between the first andsecond inputs of the comparator 42. The control module resistor 44 andthe control module capacitor 46 cooperatively form a resistor-capacitorcircuit (RC circuit) to output a delayed voltage signal to the secondinput of the comparator 42.

The control module capacitor 46 may be connected to the second input ofthe comparator 42. The control module capacitor 46 and the controlmodule resistor 44 cooperatively form an RC circuit to output a delayedvoltage signal to the second input of the comparator 42.

The memory element 48 is connected between the output node of thecomparator 42, and the first grounding switch 20 and the inverter 50.The memory element may include logic table XOR gates, a latch, aflip-flop, or the like and operates according to a state machine 200described in more detail below. The memory element 48 eliminates theneed for timing circuits for identifying which grounding switch isinitially closed and ensuring the correct grounding switch is closed foreach phase of operation.

The inverter 50 is connected to the memory element 48 and connectedbetween the first grounding switch 20 and the second grounding switch22. The inverter 50 ensures the state of the second grounding switch 22is opposite the state of the first grounding switch 20. Importantly, theinverter 50 ensures the first and second variable capacitors 16, 18 arenever concurrently grounded or “active”.

Turning to FIG. 3 and with reference to FIGS. 1 and 2, use of theelectrostatic energy harvester 10 will now be described. Initially,either the first grounding switch 20 or the second grounding switch 22is closed and hence either the first variable capacitor 16 or the secondvariable capacitor 18 is connected (i.e., “active”) by default.Identifying via the state machine 200 of the switching control module 24which grounding switch is initially closed (and hence which variablecapacitor is initially active) will be discussed in more detail below.For the sake of discussion, assume the first variable capacitor 16 isinitially active.

An initial electrical energy investment (i.e., a pre-charge) will beprovided from the electrical energy storage component 12 to the highnode 32, and hence to the dual variable capacitor 34, via the firstdiode-connected transistor, as shown in block 100. The initialelectrical energy investment creates an electric field across the firstvariable capacitor 16 (since it is active).

External vibrations induce oscillations of the high node 32 of the dualvariable capacitor 34. Depending on the phase of oscillation,capacitance of the first variable capacitor 16 will initially beincreasing or decreasing. Assuming capacitance of the first variablecapacitor 16 is initially decreasing (e.g., from C_(max)), voltage atthe high node 32 is increasing. Identifying whether capacitance isinitially increasing or decreasing via the state machine 200 of theswitching control module 24 will be described in more detail below.Voltage of the high node 32 then clamps to the voltage of the electricalenergy storage component 12 via the first diode-connected transistor 26,as shown in block 102. That is, when the voltage of the high node 32 isone diode drop above the clamping voltage, the voltage of the firstvariable capacitor 16 may be held constant due to the presence of thesecond diode-connected transistor. At this point, capacitance may be atC_(mid):

$C_{{mid},r} = {\frac{V_{clamp} - V_{D,{PCH}}}{V_{clamp} + V_{D,H}}*{C_{\max}.}}$

As capacitance of the first variable capacitor 16 decreases (e.g., fromC_(mid) to C_(min)), electrical energy may be harvested from the firstvariable capacitor 16 to the electrical energy storage component 12 (asshown in block 104) via the second diode-connected transistor 30:

E _(harvest)=(C _(mid,r) −C _(min))*(V _(clamp) +V _(D,H))².

A remnant electrical energy may remain at the high node 32.

As the oscillation continues, capacitance of the first variablecapacitor 16 begins to increase while voltage of the first variablecapacitor 16 begins to decrease. The comparator 42 continuously receivesa delayed voltage signal via the control module resistor 44 and thecontrol module capacitor 46 and a current voltage signal. The comparator42 compares the delayed voltage signal to the current voltage signal andoutputs a decreasing voltage signal to the switching control module 24.In this way, the switching control module 24 recognizes the decrease involtage of the first variable capacitor 16 and hot switches to thesecond variable capacitor 18, as shown in block 106. In other words, theswitching control module 24 deactivates the first variable capacitor 16and activates the second variable capacitor 18.

Voltage at the high node 32 drops dramatically once the first and secondvariable capacitors 16, 18 are switched. The dramatic voltage dropyields:

$V_{low} = {{{\frac{C_{\min}}{C_{\max}}*V_{clamp}} + {V_{D,H}.C_{{mid},r,{new}}}} = {\frac{V_{low}}{V_{clamp} - V_{D,{PCH}}}*{C_{\max}.}}}$

The voltage at the high node 32 continues to drop until it is one diodedrop below the clamping voltage due to the presence of the firstdiode-connected transistor 26.

A subsequent electrical energy investment may then be provided from theelectrical energy storage component 12 to the high node 32 (and hence tothe active variable capacitor) via the first diode-connected transistoronce the voltage at the high node 32 reaches a low voltage, as shown inblock 108. The subsequent electrical energy investment may be less thanthe initial electrical energy investment due to the remnant electricalenergy remaining at the high node 32 from the previous electrical energyharvesting. Specifically, if V_(low) is less than V_(clamp)−V_(D,PCH),then the electrical energy storage component 12 invests:

E _(invest)=0.5*(C _(max))*(V _(clamp) −V _(D,PCH) V _(low))².

Blocks 102-108 may be repeated, alternating between the second variablecapacitor 18 and the first variable capacitor 16. In this way,electrical energy is invested and harvested twice per vibration cycle.Exemplary values include the following: V_(clamp)=10V;V_(D,PCH)=V_(D,H)=3V; C_(min)=50 pF; C_(max)=250 pF; and C_(mid,r)=134.6pF. E_(harvest)=14.3 nJ. A dramatic voltage drop of V_(low)=2.5V.Because energy is invested and harvested twice per vibration cycle,E_(total)=14.3 nJ−2.5 nJ+14.3 nJ−2.5 nJ=23.6 nJ.

Turning to FIG. 4, operation of the switching control module 24 will nowbe described in more detail. The comparator output is always initiallylogic low (denoted by V_(comp)=0) because the control module capacitor46 for the signal delay RC circuit initially has no charge. This meansthe non-delayed input to the comparator 42 is of value equal to orgreater than the delayed input. Taking the state of the first groundingswitch 20 to be unknown, there are four potential initial conditions: 1)the first grounding switch 20 is closed and the first variable capacitor16 has decreasing capacitance; 2) the first grounding switch 20 isclosed and the second variable capacitor 18 has decreasing capacitance;3) the first grounding switch 20 is open and the first variablecapacitor 16 has decreasing capacitance; and 4) the first groundingswitch 20 is open and the second variable capacitor 18 has decreasingcapacitance. The first two conditions can be grouped into a firstscenario in which the first grounding switch 20 is closed and a secondscenario in which the first grounding switch 20 is open.

In scenario 1, the first grounding switch 20 is closed such that a gatevoltage or primary XOR logic state of the memory element 48 is logichigh (denoted by V_(gate)=1). With V_(comp)=0 and V_(gate)=1, asecondary XOR logic state of the memory element 48 is logic high(denoted by V_(mid)=1), as shown in block 202 of the state machine 200.Either the first variable capacitor 16 has decreasing capacitance inwhich case the voltage at the high node 32 will increase until thecapacitance of the first variable capacitor 16 begins to increase or thesecond variable capacitor 18 has decreasing capacitance in which casethe high node voltage will decrease immediately. Either way, when thehigh node voltage begins to decrease, V_(comp) will change from 0 to 1.With V_(comp)=1 and V_(mid)=1, V_(gate) will change from 1 to 0, asshown in block 204 of the state machine 200, thereby opening the firstgrounding switch 20. The inverter 50 in turn closes the second groundingswitch 22. The high node voltage then begins to increase, changingV_(comp) from 1 to 0. With V_(gate)=0 and V_(comp)=0, V_(mid) changesfrom 1 to 0, as shown in block 206 of the state machine 200. When thesecond variable capacitor 18 begins to increase in capacitance, voltageat the high node begins to drop such that V_(comp) changes from 0 to 1.With V_(comp)=1 and V_(mid)=0, V_(gate) changes from 0 to 1, as shown inblock 208 of the state machine 200. With V_(comp)=1 and V_(gate)=1,V_(mid) remains at 0. When the high node voltage begins to increase,V_(comp) changes from 1 to 0. With V_(comp)=0 and V_(gate)=1, V_(mid)changes from 0 to 1, as shown in block 202 of the state machine. Thestate machine 200 continues to cycle through blocks 202-208.

Scenario 2 is the same as scenario 1 but with a different startingpoint. In scenario 2, the first grounding switch 20 is open such thatV_(gate)=0. With V_(gate)=0 and V_(comp)=0, V_(mid)=0, as shown in block206 of the state machine 200. Either variable capacitor can havedecreasing capacitance which only affects how soon the switching controlmodule 24 changes state. The high node voltage eventually begins todecrease, causing the comparator output V_(comp) to change from 0 to 1.With V_(comp)=1 and V_(mid)=0, V_(gate) changes from 0 to 1, as shown inblock 208 of the state machine 200, thus closing the first groundingswitch 20. The inverter 50 in turn opens the second grounding switch 22.With V_(comp)=1 and V_(gate)=1, V_(mid) remains at 0. The high nodevoltage then begins to increase as the first variable capacitor 16decreases in capacitance, thereby changing V_(comp) from 1 to 0. WithV_(comp)=0 and V_(gate)=1, V_(mid) changes from 0 to 1, as shown inblock 202 of the state machine 200. The state machine 200 continues tocycle through blocks 202-208.

A somewhat condensed state machine 210 is expressed in FIG. 5. If thefirst grounding switch 20 is closed (block 212), the high voltage nodeeventually begins to decrease in voltage, represented by V_(comp)=1.This changes V_(gate) from 1 to 0, as shown in block 214, therebyopening the first grounding switch 20. The inverter 50 in turn opens thesecond grounding switch 22. The next time the high voltage node beginsto decrease in voltage, V_(gate) changes from 0 to 1 as shown in block212, thereby closing the first grounding switch 20. The inverter 50 inturn opens the second grounding switch 22. This condensed state machinecan be implemented by a flip flop, such as the flip flop 216 shown inFIG. 6.

The above-described electrostatic energy harvester 10 provides severaladvantages. For example, the dual variable capacitor 34 enableselectrical energy to be harvested twice per vibration cycle. Hotswitching the first and second variable capacitors 16, 18 eliminatesenergy losses by keeping remnant energy at the dual variable capacitor34. The first and second diode-connected transistors 26, 30 eliminatethe need for control circuitry between the electrical energy storagecomponent 12 and the dual variable capacitor 34. The first and seconddiode-connected transistors 26, 30 also eliminate high current leakage,dual batteries, and energy harvesting limits based on harvesting timelost while variable capacitor voltage increases. The switching controlmodule 24 (for controlling hot switching of the first and secondvariable capacitors 16, 18) reduces complexity of variable capacitorcontrol circuitry.

Turning to FIG. 7, another embodiment of the invention is anelectrostatic energy harvester 300 broadly comprising an electricalenergy storage component 302, an electrical energy transfer stage 304,an electrical energy transfer stage control module 306, a first variablecapacitor 308, a second variable capacitor 310, a first grounding switch312, a second grounding switch 314, and a switching control module 316.The electrostatic energy harvester 300 is similar to the electrostaticenergy harvester 10 described above except various electrical energytransfer stages (such as electrical energy transfer stage 400 describedbelow) and various electrical energy transfer stage control modules maybe used.

The electrical energy storage component 302 is substantially similar tothe electrical energy storage component 12 described above. That is, theelectrical energy storage component 302 may be connected to theelectrical energy transfer stage 304 and may be a battery, a largecharged capacitor, a super-capacitor, or any other suitable electricalenergy storage device. The electrical energy storage component 302 maybe configured provide an initial electrical energy investment andsubsequent electrical energy investments to the first and secondvariable capacitors 308, 310 and to store electrical energy harvestedfrom the first and second variable capacitors 308, 310.

The electrical energy transfer stage 304 may be connected between theelectrical energy storage component 302 and the first and secondvariable capacitors 308, 310. The electrical energy transfer stage 304dictates electrical energy transfer between the electrical energystorage component 302 and the first and second variable capacitors 308,310. The electrical energy transfer stage 304 may include switches orother electrical or electronic components (see the exemplary electricalenergy transfer stage 400 described below). Unlike the electrical energytransfer stage 14 described above, the electrical energy transfer stage304 may require the electrical energy transfer stage control module 306.

The electrical energy transfer stage control module 306 may be connectedto the electrical energy storage component 302 and/or to the electricalenergy transfer stage 304. The electrical energy transfer stage controlmodule 306 includes switches or other electrical or electroniccomponents for controlling the electrical energy transfer stage.

The first variable capacitor 308 may be connected to the electricalenergy transfer stage 304 via a high node 318 and to the first groundingswitch 312 via a low node. The high node 318 may be a shared node thatthe first variable capacitor 308 shares with the second variablecapacitor 310. In one embodiment, the first variable capacitor 308 maybe part of an in-plane microelectromechanical (MEM) dual variablecapacitor 320 with the high node 318 (i.e., the shared node) being anoscillating/vibrating (i.e., “floating”) electrode and the low nodebeing a stationary comb.

The second variable capacitor 310 may be connected to the electricalenergy transfer stage 304 via a high node and to the second groundingswitch 314 via a low node. The high node of the second variablecapacitor 310 may be the shared node (high node 318) described above. Inone embodiment, the second variable capacitor 310 may be part of thein-plane MEM dual variable capacitor 320 described above, with the lownode of the second variable capacitor 310 being a stationary comb.

The first grounding switch 312 may be connected between the low node ofthe first variable capacitor 308 and a grounding plane or groundingnode. The first grounding switch 312 may be a relay or any othersuitable electronic switch.

The second grounding switch 314 may be connected between the low node ofthe second variable capacitor 310 and a grounding plane or groundingnode. The second grounding switch 314 may be a relay or any othersuitable electronic switch. In one embodiment, the first and secondgrounding switches 312, 314 are never concurrently closed and hence thefirst and second variable capacitors 308, 310 are never concurrentlygrounded or “active”.

The switching control module 316 timely switches between the first andsecond variable capacitors 308, 310. The switching control module 316may be substantially similar to the switching control module 24described above and thus will not be discussed further.

Turning to FIG. 8, an electrical energy transfer stage 400 constructedin accordance with another embodiment of the invention is shown. Theelectrical energy transfer stage 400 broadly comprises an energyinvestment switch 402, an inductor 404, a grounding switch 406, and adiode-connected transistor 408. The electrical energy transfer stage 400can be used as the electrical energy transfer stage 304 of theelectrostatic energy harvester 300 described above.

The energy investment switch 402 may be connected between an electricalenergy storage component 410 (described below) and the grounding switch406. The energy investment switch 402 may be configured to be closed byelectrical energy transfer stage control module 412 (described below)during an electrical energy investment phase so that electrical energymay transferred from the electrical energy storage component 410 to avariable capacitor (or to multiple variable capacitors). The energyinvestment switch 402 may be configured to be opened by the electricalenergy transfer stage control module 412 during an electrical energyharvesting phase.

The inductor 404 may be connected between the energy investment switch402 and a high node of the variable capacitor(s). The inductor 404 slowsdown the flow of charge from the electrical energy storage component 410to the variable capacitor(s), thus reducing energy loss in the form ofheat. The inductor 404 can be omitted at the cost of (additional) energylosses in the form of heat resulting in a less efficient electrostaticenergy harvester.

The grounding switch 406 may be connected between the energy investmentswitch 402 and the inductor 404, and ground node or ground plane. Thegrounding switch 406 may be configured to be opened by the electricalenergy transfer stage control module 412 during the energy investmentphase and closed by the electrical energy transfer stage control module412 during the energy harvesting phase.

The diode-connected transistor 408 may be connected to the electricalenergy storage component 410 via its anode and to the variablecapacitor(s) via its cathode. The second diode-connected transistor 30may be a depletion mode gallium nitride transistor.

The electrical energy storage component 410 may be connected via itshigh node to the electrical energy transfer stage 400 and to theelectrical energy transfer stage control module 412. The electricalenergy storage component 410 may be a battery, a large chargedcapacitor, a super-capacitor, or any other suitable electrical energystorage device. The electrical energy storage component 410 may beconfigured provide an initial electrical energy investment andsubsequent electrical energy investments to the variable capacitor(s)and to store electrical energy harvested from the variable capacitor(s).

The electrical energy transfer stage control module 412 may be connectedto the electrical energy storage component 410 and to the electricalenergy transfer stage 400. Specifically, outputs of the electricalenergy transfer stage control circuit 412 may be connected to the energyinvestment switch 402 and the grounding switch 406, while inputs of theelectrical energy transfer stage control circuit 412 may be connected tothe electrical energy storage component 410, between the energyinvestment switch 402 and the inductor 404, and between the inductor 404and the variable capacitor(s). The electrical energy transfer stagecontrol module 306 includes electrical or electronic components forcontrolling the electrical energy transfer stage and more specifically,for cycling between electrical energy investment from the electricalenergy storage component 410 to the variable capacitor(s) during theelectrical energy investment phase and electrical energy harvesting fromthe variable capacitor(s) to the electrical energy storage component 410during the electrical energy harvesting phase.

Use of the electrical energy transfer stage 400 may be substantiallysimilar to use of the electrical energy transfer stage 14 describedabove, except an electrical energy transfer stage control module 412 isused to control electrical energy transfer between the electrical energystorage component 410 and the variable capacitor(s). Specifically, theelectrical energy transfer stage control module 412 closes the energyinvestment switch 402 and opens the grounding switch 406 during theelectrical energy investment phase so that electrical energy maytransferred from the electrical energy storage component 410 to thevariable capacitor(s). The electrical energy transfer stage controlswitch 406 opens the energy investment switch 402 and closes thegrounding switch 406 during the electrical energy harvesting phase sothat electrical energy may be transferred from the variable capacitor(s)to the electrical energy storage component 410.

Although the invention has been described with reference to theembodiments illustrated in the attached drawing figures, it is notedthat equivalents may be employed and substitutions made herein withoutdeparting from the scope of the invention as recited in the claims.

Having thus described various embodiments of the invention, what isclaimed as new and desired to be protected by Letters Patent includesthe following:
 1. An electrostatic energy harvester comprising: anelectrical energy source configured to supply an initial electricalenergy investment and subsequent electrical energy investments; anelectrical energy transfer stage connected to the electrical energysource and configured to control energy transfer to and from theelectrical energy source; a first variable capacitor connected to theelectrical energy transfer stage; and a second variable capacitorconnected to the electrical energy transfer stage, the first and secondvariable capacitors being out of phase from each other, the secondvariable capacitor being configured to retain remnant electrical energyunharvested from the first variable capacitor in a first energyharvesting phase, the first variable capacitor being configured toretain remnant electrical energy unharvested from the second variablecapacitor in a second energy harvesting phase so that the subsequentelectrical energy investments are less than the initial electricalenergy investment.
 2. The electrostatic energy harvester of claim 1, thefirst variable capacitor and the second variable capacitor sharing acommon electrode, the first variable capacitor and the second variablecapacitor each including a grounding electrode.
 3. The electrostaticenergy harvester of claim 2, the first variable capacitor and the secondvariable capacitor cooperatively forming a micro-electro-mechanical(MEM) in-plane variable capacitor.
 4. The electrostatic energy harvesterof claim 2, the first variable capacitor and the second variablecapacitor being 180 degrees out of phase from each other.
 5. Theelectrostatic energy harvester of claim 2, the first variable capacitorand the second variable capacitor being configured to be hot switched.6. The electrostatic energy harvester of claim 2, further comprising afirst switch connected to the first variable capacitor and a secondswitch connected to the second variable capacitor, the first switch andthe second switch being configured such that the first variablecapacitor and the second variable capacitor are never concurrentlyactive.
 7. The electrostatic energy harvester of claim 6, the firstvariable capacitor and the second variable capacitor being configuredsuch that one of the first variable capacitor and the second variablecapacitor is active and if a voltage at the common electrode begins todecrease, the active variable capacitor is deactivated and the othervariable capacitor is activated.
 8. The electrostatic energy harvesterof claim 7, further comprising a switching control module configured tocontrol the first switch and the second switch.
 9. The electrostaticenergy harvester of claim 1, further comprising an electrical energytransfer stage control module configured to control the electricalenergy transfer stage.
 10. The electrostatic energy harvester of claim1, the electrical energy transfer stage including two diode-connectedgallium nitride transistors.
 11. A method of harvesting electrostaticenergy, the method comprising the steps of: receiving an initialelectrical energy investment at a first variable capacitor, the firstvariable capacitor being active; receiving mechanical energy at thefirst variable capacitor so as to convert at least a portion of themechanical energy to electrical energy stored on the first variablecapacitor; transferring some of the electrical energy stored on thefirst variable capacitor to an electrical energy source; deactivatingthe first variable capacitor and activating the second variablecapacitor such that some of the electrical energy stored on the firstvariable capacitor is retained on the second variable capacitor;receiving a subsequent electrical energy investment at the secondvariable capacitor, the subsequent electrical energy investment beingless than the initial electrical energy investment; receiving mechanicalenergy at the second variable capacitor so as to convert at least aportion of the mechanical energy imparted to the second variablecapacitor to electrical energy stored on the second variable capacitor;and transferring some of the electrical energy stored on the secondvariable capacitor to the electrical energy source.
 12. The method ofclaim 11, the first variable capacitor and the second variable capacitorsharing a common electrode, the first variable capacitor and the secondvariable capacitor each including a grounding electrode, the methodfurther comprising the step of vibrating the common electrode.
 13. Themethod of claim 12, wherein the step of vibrating the common electrodeincludes vibrating the common electrode such that the first variablecapacitor and the second variable capacitor are 180 degrees out of phasefrom each other.
 14. The method of claim 12, wherein the step ofdeactivating the first variable capacitor and activating the secondvariable capacitor is performed when a voltage at the common electrodebegins to decrease.
 15. The method of claim 11, wherein the step ofdeactivating the first variable capacitor and activating the secondvariable capacitor includes hot switching the first variable capacitorand the second variable capacitor.
 16. The method of claim 11, whereinthe step of deactivating the first variable capacitor and activating thesecond variable capacitor includes the steps of opening a first switchconnecting the first variable capacitor to a grounding node and closinga second switch connecting the second variable capacitor to a groundingnode.
 17. The method of claim 16, wherein the steps of opening the firstswitch and closing the second switch include controlling the firstswitch and the second switch via a switching control module.
 18. Themethod of claim 17, further comprising the step of ensuring the firstswitch and the second switch are never concurrently closed via aninverter.
 19. The method of claim 12, wherein the steps of receiving theinitial electrical energy investment, receiving the subsequentelectrical energy investment, harvesting some of the electrical energystored on the first variable capacitor and harvesting some of theelectrical energy stored on the second variable capacitor includeelectrical energy supply and transfer control via an electrical energytransfer stage control module.
 20. A method of harvesting electrostaticenergy, the method comprising the steps of: receiving an initialelectrical energy investment at a first variable capacitor via anelectrical energy transfer stage, the first variable capacitor beingactive and sharing a common electrode with a second variable capacitor,the first variable capacitor and the second variable capacitor eachincluding a grounding electrode, the first variable capacitor and thesecond variable capacitor cooperatively forming an in-plane MEM variablecapacitor; receiving mechanical energy at the first variable capacitorso as to convert at least a portion of the mechanical energy toelectrical energy stored on the first variable capacitor; transferringsome of the electrical energy stored on the first variable capacitor toan electrical energy source via the electrical energy transfer stage;deactivating the first variable capacitor and activating the secondvariable capacitor via a switching control module such that some of theelectrical energy stored on the first variable capacitor is retained onthe second variable capacitor; receiving a subsequent electrical energyinvestment at the second variable capacitor via the electrical energytransfer stage, the subsequent electrical energy investment being lessthan the initial electrical energy investment; receiving mechanicalenergy at the second variable capacitor so as to convert at least aportion of the mechanical energy imparted to the second variablecapacitor to electrical energy stored on the second variable capacitor;and transferring some of the electrical energy stored on the secondvariable capacitor to the electrical energy source via the electricalenergy transfer stage.